Thermal detectors include a sensor that absorbs light energy and then transduces the resulting heat into a useful electrical signal related to the amount or type of light absorbed. Perhaps the most prominent current thermal detectors include microbolometers, which absorb light across a broad band of the infrared, usually the mid-wave infrared (MWIR, corresponding to wavelengths of roughly 3 microns to 5 microns) or long wave infrared (LWIR, roughly 8 microns to 14 microns) and then convert heat into a change in resistance. These devices are very popular in commercial uncooled imaging cameras. Their basic structure includes a small micromachined sensor plate connected to an underlying substrate by thin support beams. The support beams have a low thermal conductance so that large increases in the temperature of the sensor plate occur with small amounts of absorbed light. The sensor plate includes a resistor made of a material with a high magnitude temperature coefficient of resistance (TCR). One common TCR material in use is vanadium oxide, originally developed for microbolometers in the 1980's. A pulsed or continuous bias current is applied to the resistor and the absorbed light energy can be measured through the voltage response. Some other common thermal detector technologies include thermoelectric detectors where the heat from light is converted into a voltage using the Seebeck effect, and pyroelectric detectors where heat from absorbed light induces a voltage signal via a change in the internal polarization of a ferroelectric material.
There are a variety of noise sources that can limit the performance of a thermal detector. For a biased single-pixel detector, the most important of these include Johnson noise, 1/f noise, and thermal noise. Thermal noise originates from the fluctuations in the quanta of energy transferred to and from the detector. These quanta can take the form of either phonons if solid-state conduction dominates the heat transfer or photons if radiation dominates. Traditionally, radiation heat transfer has been considered the fundamental noise limit because even if all of the other noise sources are reduced by technological innovations, the photon fluctuations still remain due to Planck's Law.
Current broadband thermal detector devices are beginning to perform in regimes where radiation noise must be considered. An example of this is shown in U.S. Publication Nos. 2002/0139933 and 2001/0028035 which describe a microbolometer with low thermal conductance supports. In one embodiment, the authors propose to select a material for the backside of their detector which has a radiation emission lower than many other materials.
The radiation limit is more forgiving than has been previously considered. Because radiation heat transfer is directly proportional to emissivity and emissivity is identical to absorption through Kirchoff's Law, a low absorption structure will interact with the radiation of the background much less than a normal thermal detector, which is usually optimized for high absorption. For a typical detector, this does nothing to improve performance because the received optical signal is reduced by the same amount. If the signal light can be coupled into the sensor of the detector at near 100% efficiency while maintaining a low absorption for the rest of the background, then the traditional thermal radiation noise could be reduced by multiple orders of magnitude.
Most thermal detectors operate with broad bands. Many current devices have a reflector beneath them to allow light transmitted through the device another chance to be absorbed. The gaps between the bolometer and substrate in these structures are roughly a quarter-wave length to enhance coupling across a wide band. In a sense, the entire bolometer plus substrate can be though of as a highly absorbing distributed “mirror.” A variety of narrowband detectors using integrated external etalons have been proposed, perhaps the most advanced of which are described by in U.S. Pat. Nos. 7,015,457 and 5,550,373 to Cole et al. A multispectral bolometer concept that uses scattering rather than interference to distribute different wavelengths is described in U.S. Pat. No. 5,629,521 to Lee et al. Detectors with integrated filtering are described in U.S. Pat. No. 5,589,689 to Koskinen, U.S. Publication No. 2005/0017177 to Tai et al., and U.S. patent application Ser. No. 11/805,240 to Talghader et al.